In the small appliance and tool field, such as hand held mixers, knives and drills, a universal series motor is used to operate one or more tool elements that move to accomplish a specific task. In such units, it is quite important to be able to vary the speed output of the operating tool element. In the control of such appliances, especially one powered by a low or fractional horsepower universal series motor, it is possible to vary the speed of the motor by varying the effective field coil windings.
Thus, the typical universal motor has a field coil and a wound armature connected in a series power circuit via commutator means carried on the armature. Selective taps are taken off the field coil windings and are connected by lead wires to a switching means for bypassing some of the field coil windings. This changes the number of windings of the effective field coil and thus the amperage conducted in the power circuit, the lowest number of field coil windings providing the greatest motor speed and the highest number of field coil windings providing the lowest motor speed. However, this requires the use of a multi-contact switch and further only provides for a step-by-step variation in the motor speed depending on the number of separate taps and positions available on the switch. Further, the cost of the switch and the required lead wires and insulation as well as the cost of the necessary steps of stripping insulation from the lead wires, soldering and applying insulation to connections typically adds appreciably to the overall cost of the unit.
Another problem area is in the commutator means and particularly the means for biasing the commutator brush against the rotating commutator contacts while preventing the biasing means from ever engaging the commutator contacts should the brush become overly worn and/or dislodged from its guides. In this regard, the commutator brush is typically formed of a carbon alloy or compound and is relatively soft when compared to biasing means generally formed of a hard conductive metal. Thus, if the biasing means were allowed to engage the rotating commutator contacts, the commutator contacts could be torn up and damaged.
The appliance and tool field is highly competitive and every cost-saving gives an edge to the supplier. The manner of assembly thereby becomes quite critical, where the number of separate steps needed to assemble the appliance as well as the number of separate individual pieces needed in the assembly can critically add to the cost of manufacture to the extent that the appliance or tool may be priced out of the market. Thus, complexity of the construction is detrimental while simplicity of both the construction and the manner of assembly are highly beneficial.
Lastly, the cost saving in providing a modular motor construction for small appliances whereby various models may be provided with different numbers of operating speeds by merely utilizing a different speed control detent with the same modular motor is obviously considerable.